MDMX is a homolog of MDM2 and is critical for regulating p53 function during mouse development. MDMX level is regulated by MDM2-mediated poly-ubiquitination, which results in its accelerated degradation after DNA damage or expression of ARF. In this report. we demonstrate that MDMX can be modified by conjugation to SUMO-1 both in vivo and in vitro. We found that double Mutation of two lysine residues. K254 and K379, abrogated MDMX sumoylation in vivo. Experiments using the sumoylation-deficient MDMX mutant showed that it undergoes normal ubiquitination and degradation h MDM2. normal nuclear translocation and degradation after DNA damage, and inhibits p53 with wild type efficiency. Therefore. sumoylation is not required for several activities of MDMX under our assay conditions. (c) 2005 Elsevier Inc. All rights reserved.